In the past, motor-generators (hereinafter referred to as "machines") have been used in specific applications based upon their operating characteristics and size. For instance, the electrical type may be alternating current induction, synchronous, or series; or direct current series, permanent magnet, shunt, or compound; or universal. The speed variability may be classified as constant speed, varying speed, adjustable speed, adjustable varying speed, or multispeed. The mechanical protection and cooling may be open or totally enclosed, or it may be ventilated, water cooled, air cooled, fan cooled, etc. Through its unique structure, the machine of the present invention has combined many of the optimum operating characteristics of the different types of motors. It combines synchronous and induction characteristics in a solid rotor machine that utilizes permanent magnets. It may be used in different environments, with alternating current or direct current, and it provides a constant torque output through its speed range, which includes very high speeds due to its solid rotor construction.
The machine of the present invention is based upon the same laws of electromagnetism utilized in conventional machines, but is different in its configuration and mode of operation. For instance, most prior generators are characterized by relative motion between a coil of wire and a magnetic field such that the wire cuts through the lines of force of the magnetic field to generate electricity in the coil. This requires electrical windings on a rotor, the use of slip rings or brushes to accommodate the rotation of the windings or to commutate the flow of electricity, and the maintenance of a rotating electrical connection. Some of the problems associated with such a design include the continuous attention required by slip rings or brushes, the serious operational hazards arising from the continuous arcing and sparking as the rotor contacts move past the stationary brush. Slip rings and brushes also degrade operational efficiency through electric power loss from the electrical resistance of the brush contact, and through mechanical friction loss from the drag of the brushes on the rotor. The presence of windings on the rotor also significantly increases the weight or mass of the rotor, necessitating slower rotational speeds and more energy from the prime mover. Still further, the constant rotation, heating, and magnetic forces exerted on the coils and their insulation causes them to fatique, crack, degrade, and ultimately fail with time.
Some prior machines have functioned without slip rings or brushes, but have other inherent limitations. For example, inductor machines vary the magnetic path induction by means of a wireless toothed rotor. The field is maintained by electromagnets on the stator, and the armature coils are also wound on the stator. However, inductor machines have not found widespread acceptance since they are more bulky and less efficient than more traditional brush and slip ring machines.
More recent solutions to these problems have been found by mounting permanent magnets, rather than electromagnets, on the rotor. This eliminates the need for rotating electrical connections, saves the electrical power otherwise expended in exciting the field, lessens the amount of internal heat generation, and increases power density. However, such permanent magnet machines are limited in application since they have no viable means for controlling operation with changes in load.
Further aspects of prior machines which detract from performance include full reversal of the magnetic field throughout the machine, resulting in hysteresis loss, eddy current loss, and heat production; rotors made of different materials, which result in non-uniform expansion, mechanical stress, and eddy current loss; non-uniformity of torque with respect to speed, producing higher harmonics which reduce output quality and efficiency; and magnetic paths that operate at less than full saturation, resulting in a low energy density and an increase in bulk and weight.
The present invention incorporates the positive characteristics of many of the different machine types, including but not limited to those discussed herein. Further, it eliminates or mitigates many of the problems associated with earlier machine designs.
The efficiency and operational characteristics of the machine are improved as a result of several factors. The present invention utilizes permanent magnets and a multicomponent rotor. Rotation of the rotor induces variations in flux magnitude and flux direction (i.e. flux reversals) in the stator without reversing the flux direction in the rotor. Thus, flux reversals are limited to that portion of the stator surrounded by the stator windings. It is usually preferred that the flux magnitude in the rotor remain constant, but it may vary as necessary or desirable. The rotor routes the flux through the magnetically isolated and separate rotor components, and reverses the flux flow as it spins, even at high speed. This avoids undesirable flux cancellation and heat built up, and accomplishes a very high energy density. Substantially all of the magnetically permeable paths are fully saturated at all times, reducing the amount of iron, bulk and weight, and increasing the horse-power to weight ratio. The magnetic field is supplied by permanent magnets, eliminating the primary field current and brushes or slip rings. This also increases the power density through flux squeezing and decreases heat production. The essentially constant field strength, along with use of a properly shaped and spaced permeable portions of the rotor, results in a more uniform torque, or voltage output, that is virtually free of higher harmonic distortion. If the magnetic means is stationary and attached to the end bell, higher speeds and larger rotors are possible, ruggedness and reliability are improved, conventional design and manufacturing techniques may be utilized, and assembly and disassembly is easier. And, with the magnetic means located on the end bell, it is thermally isolated from the remainder of the magnetic circuit by the air gap, minimizing the degradation of the permanent magnets with increasing temperatures.
Generators relying upon permanent magnets for excitation and operation may suffer from a drop in voltage as current is drawn out. This problem arises from the inability to control the magnetic field to compensate for the voltage drop. However, the present invention uses its unique design and permanent magnets coupled with voltage regulation windings which are energized to increase or decrease the magnetic flux as necessary. The voltage control windings are also selectively energized to assist in suppressing voltage surges or spikes.
When operated as a motor, the machine of the present invention may be operated as a constant speed motor by adding damping windings to improve starting torque and eliminate unwanted oscillations during load changes. It may be operated as a variable speed motor by coupling it with an inverter that locks the rotor into synchrony at a low speed and brings the rotor up to high speeds as desired.
The machine may be operated as a single phase or polyphase system, making it more versatile for sophisticated application, including co-generation.
Thus, it is an object of the present invention to provide an improved motor-generator that combines the optimum operating characteristics of different types of machines.
It is a further object of the present invention to provide a machine that operates by changing the magnitude and direction of the magnetic flux through a winding on the stator without changing the direction of the flux in the rotor.
It is a further object of the present invention to provide a machine having a multipart rotor with complementary and magnetically isolated components.
It is a further object of the present invention to provide a machine with a solid rotor, and one that is capable of high speed operation.
It is a further object of the present invention to provide a machine that makes efficient use of high energy permanent magnets.
These and other objects are met by the machine of the present invention.